1. Field of the Invention
The present invention relates to current sensors, and more particularly, to a current sensor having an improved reset circuit.
2. Description of the Related Art
Within various circuit implementations, such as power supplies, there is often a need to detect a current provided at a particular point within a circuit. For example, a detected current may be used as feedback for controlling other parts of a circuit. Various techniques are presently used to sense currents within electronic circuits, but each of these techniques have shortcomings. One approach, illustrated in FIG. 1, utilizes a resistor 102 connected across the inputs of an operational amplifier 104 to provide a voltage VSENSE that may be used to determine a current 106. A low value resistor, in the range of 10 mOhms, may be implemented. However, a drawback of this approach is the high loss provided by the circuit. The high loss may be mitigated by reducing the value of the resistor 102, however, this also reduces the signal VSENSE that may be detected. While this type of circuit may be used to sense current in direct current (DC) applications, the resistor 102 has usually not been capable of being readily integrated.
Referring now to FIG. 2, a further prior art system, utilizing a Hall effect device 202, connected across the inputs of an operational amplifier 204, is illustrated. The Hall effect device 202 generates a voltage across the inputs of the operational amplifier 204, responsive to a current 206, to provide an output signal VSENSE. While this approach has a relatively low loss and may be used to detect direct current (DC), the use of the Hall effect device 202 generally provides a circuit having a higher cost. Furthermore, accuracy and noise issues are generally greater in current sensors that implement a Hall device, as the Hall voltage is a relatively small value.
With reference to FIG. 3, a current sensor that uses a magneto resistive sensor is illustrated. The magneto resistive sensor consists of a magneto resistive element 302 connected across the inputs of operational amplifier 304 to detect a current 306. The magneto resistive element 302 has the property that the resistance of the element changes with respect to the magnetic field caused by the current 306. This circuit requires the use of special technology which raises the cost of the device. Additionally, accuracy issues arise even though the current may be sensed with very low loss.
Referring now to FIG. 4, an alternative prior art technique for detecting current, through the use of a current transformer 402, is illustrated. As is shown, the current transformer 402 has a primary side 404 with a single loop and a secondary side 406 with multiple loops. A load resistance 408 is in parallel with the secondary side 406 of the transformer 402. The current transformer 402 is used to detect a current 410. The transformer 402 creates an output current equal to Ip/n, with Ip being the detected current and n being the turns ratio of the transformer 402. In this configuration, the resistance of the secondary side of the transformer is reflected to the primary side with the ratio 1/n2. While current transformers work well for detecting currents, they are large and have a medium loss level and only work with alternating current (AC) circuits.
Another method for measuring currents involves the use of a Rogowski coil. Unfortunately, the voltage induced in a Rogowski coil is very small and easily disturbed when a measured current is less than, for example, 100 Amps. However, a Rogowski current transducer has a number of advantages over the current transformer illustrated in FIG. 4. For example, the Rogowski current sensor is linear, has no core saturation effects, has a wide bandwidth and a wide measurement range and is a relatively simple structure. The Rogowski coil comprises a toroidal winding placed around a conductor being measured. The Rogowski coil is effectively a mutual inductor coupled to the inductor being measured, where the output from the winding is an EMF proportional to the rate of change of current. While the above described techniques provide an indication of a sensed current in certain applications, the techniques, as noted above, individually have a number of short comings.
What is needed is a technique for detecting a current, within, for example, a power electronic circuit, that addresses many of the shortcomings of the prior art techniques described above. It would be desirable for a current sensor configured according to the technique to be capable of being reset without adversely effecting a subsequent output signal provided by the current sensor.